The present invention relates to flowmeters for measuring pressure drops indicative of flowrates in a two-phase flowing stream.
By two-phase flow is meant a flowing stream containing gaseous and liquid phases of one or more components. Exemplary streams include wet steam, oil and gas, and air and water.
The flowmeters used to meter single-phase flow usually include a device for measuring a pressure drop in the flowing stream, which pressure drop can be correlated with the flow rate of the stream through theoretical mathematical models.
In two-phase flow however it is usually desirable to obtain values of the individual gas and liquid flowrates, W.sub.g and W.sub.f respectively. These flowrates are usually expressed in terms of total mass flow (W) and quality of flow (x), which terms are defined as: EQU W=W.sub.g +W.sub.f ; and EQU x=W.sub.g /(W.sub.g +W.sub.f)
A number of two-phase flowmeters are known in this art. Two such meters, which correlate one of the above flowrate parameters with a pressure differential measurement, are the Orifice Plate Meter and the Venturi Meter. The Orifice Plate Meter comprises an orifice mounted across the conduit carrying a two-phase flow. An accelerational pressure drop is measured across the orifice plate. A mathematical model is then used to correlate the stream's total mass flow with the accelerational pressure differential measured.
The Venturi Meter comprises a venturi placed in the conduit carrying the two-phase flow. An accelerational pressure drop across the venturi is measured and correlated by mathematical models either to the quality and total mass flow of the stream or to the quality and a dimensionless modified Collins parameter F.sub.p, which parameter will be later explained.
Both of the above two meters rely on one pressure differential measurement to evaluate parameters of two-phase flow.
Other two-phase flow meters are known, which monitor two-phase flow with two or more measurements. One such meter employs a gamma ray densitometer to make void fraction measurements and a turbine meter or drag disc to obtain a second measurement. The two measurements are correlated mathematically to indicate the individual phase mass flowrates. This metering technique is limited to a very small quality range. It is also expensive, employing a delicate gamma ray densitometer instrument. In such two-phase streams as high pressure wet steam, such instruments would not be practical.
Very recently an Orifice-Couple Flowmeter has been proposed for two-phase flow, see K. Sekoguchi et al., "Two-Phase Flow Measurements with Orifice-Couple in Horizontal Pipe Line," Bulletin of the ISME, Vol. 21, No. 162, December, 1978. The meter includes two segmental orifices or baffles in the conduit carrying the two-phase flow. Three pressure drop measurements are taken, two across the segmental orifices and the third across two of the orifices. The two individual pressure drops and the sum pressure drop are then correlated, by a model specific to this system, to the gas and liquid flowrates. This metering system appears to give very good results. A minor disadvantage to the system is that the data is not presented in a dimensionless form. Consequently, performances for different systems are difficult to predict.
Although not used for the purpose of flow metering, twisted tape swirl generators have been used in both single and two-phase flow to enhance heat transfer in heat exchangers. In these systems a tape, having a width equal to the diameter of the conduit carrying the two-phase flow, is twisted into the conduit. The twisted tape induces a swirled flow, otherwise termed annular flow. Annular flow is a two-phase flow pattern characterized by an annular film of liquid travelling along the inner wall of the tube with the gaseous flow moving through the centre core of the tube at a much larger velocity. Users of these generators have noticed a frictional pressure drop across the twisted tape. A frictional pressure drop model for gas-liquid flow through a twisted tape has been developed, see G. S. R. Narasimhamurty et al., "Effect of Turbulence Promoters in Two-Phase Gas Liquid Flow in Horizontal Pipes," Chemical Engineering Science, Vol. 24, 1969, p. 331.